Recently, there has been an increasing interest in offshore hydrocarbon production. Hydrocarbon wells may be located many miles from shore sites and in water depths reaching down to several thousand meters. Subsea pipelines may be used for transporting hydrocarbons from an offshore well to a production vessel or to an onshore site, or may be used for transporting hydrocarbons between different onshore sites separated by an offshore section.
In deep waters, the water temperature is relatively low. The water temperature may, for example, be between about −1 and +4° C. When hydrocarbons are produced from a subsea well, the hydrocarbons may include a fraction of water, and the hydrocarbons will cool significantly upon reaching the seabed. This may lead to the formation of hydrates that may be a combination of pressurized hydrocarbon gas with water. This combination may, at low temperatures, form a hydrate that is a solid material. Hydrates may restrict the flow within a pipeline, or may even completely plug the pipeline.
Methods that use chemicals for preventing hydrate formation are known in the art. Another method that is more effective is the increase of the temperature of the pipeline, for example, by using direct electric heating (DEH). Such DEH system is, for example, known from WO 2004/111519, which uses a subsea single phase cable that is attached to two sides of a steel pipeline. A 50/60 Hz AC current is passed through the cable and the pipeline, and the pipeline is heated due to electric resistance.
The power source may be located at an onsite location or on board of a production vessel, and an example of such power source is described in WO 2010/031626. The pipeline section to be heated is a single phase load on the power supply arrangement.
The problem of conventional systems is that the conventional systems are generally restricted to rather short distances between the pipeline section to be heated and the power supply. Also, the length of the pipeline to be heated is only very limited. Long step-out distances may thus generally not be realized. Such systems may lack any way of controlling the heating. Also, there are significant losses of electric energy along the subsea cable to the pipeline, and the subsea cable itself is a very cost intensive product.
It is desirable to enable the heating of pipeline sections located further away from the main power supply and the heating of longer pipeline sections. It is desirable to reduce currents in the cable supplying the electric power to the pipeline section to be heated, and to provide a fault tolerant heating system. Also, the costs involved in such system should be reduced, and the efficiency should be increased.